It is sometimes desirable to freeze the television picture. A frozen picture, for example, may be transmitted over a narrow bandwidth channel to a remote location. Or a particular moment in a sequence of action being transmitted may be frozen on the TV screen.
The basic approach taken to the production of a still picture in a TV receiver is to store either a field or a frame of video signal in a memory device. Once a field or a frame of video signal is stored, one can produce a frozen picture by stopping or inhibiting the memory writing operation and by repeatedly displaying the contents of the memory on the television screen.
The processing of the composite video signal CVS before its storage in the memory can vary in the amount of Y/C separation performed on the incoming signal. A notch filter may be used in the luminance (Y) channel of the TV receiver to remove the chrominance component therefrom. A bandpass filter may be employed in the chrominance (C) channel to remove the low frequency luminance component from the composite video signal CVS. After the composite video signal CVS is processed in this manner, the component signals CS are then stored in the respective component memories - so that they may be processed electronically to produce special effects (such as - still picture, zoom, pix-in-pix, etc).
It will be noted that the Y/C separation in the notch-filtered and bandpass-filtered video signals is incomplete or partial in the high frequency region of the video signal frequency spectrum. This is so because the luminance and chrominance components of the composite video signal CVS share the upper portion (i.e., 2.5 to 4.1 MHz) of the frequency spectrum.
Returning to the various techniques for the production of a still picture, one possible approach is the freeze frame approach. A frame of video signal can be stored in a frame memory, and read out repeatedly to generate two interlaced fields on the TV screen. The display of two interlaced fields in this manner may produce motion-induced artifacts (i.e., causes moving portions of the picture to jitter back and forth when interfield motion is present).
Another approach to the production of still pictures is to store a field of video signal in a field memory, and to display the same field of stored signal in both of the interlaced fields (i.e., produce two identical interlaced, non-interpolated or real fields). One problem with this approach is that the diagonal edges of the picture on the TV screen appear jagged. Another drawback is that the two fields have unequal spatial offsets. One field has zero offset, and the other field has a half-line offset. A half-line offset means that the signal elements are displayed on the TV screen at positions which are spaced from their true relative positions by one-half of the line spacing.
Still another approach is to display two different interlaced fields--each produced from the same stored field of video signal. The stored field is displayed as one of the two interlaced fields. The second interlaced field is generated by averaging or interpolating two adjacent lines of the stored field to produce a line that is spatially located half-way between the two lines. This approach has a number of disadvantages. The diagonal edges of the TV picture still appear somewhat jagged as in the previous approach. A further disadvantage of this approach is that the two fields may have unequal Y/C separation.
In accordance with this invention, an apparatus is disclosed for providing two non-identical fields suitable for interlaced display, and in a manner that overcomes the afore-mentioned deficiencies in the production of a freeze picture. In one embodiment of the present invention, one of the interlaced fields is generated by adding three-fourths of a line-delayed television signal L.sub.1 to one-fourth of a non-delayed television signal L.sub.2 (i.e., F.sub.1 =3/4L.sub.1 +1/4L.sub.2). The other interlaced field is generated by adding one-fourth of the line-delayed television signal L.sub.1 to three-fourths of the non-delayed television signal L.sub.2 (i.e., F.sub.2 =1/4L.sub.1 +3/4L.sub.2). The first and second interpolated signals F.sub.1 and F.sub.2 are repeatedly displayed on the TV screen during odd and even fields respectively to generate a frozen picture.
Pursuant to another embodiment of the instant invention, a frequency responsive interpolating apparatus is disclosed. For low frequency luma signal, the apparatus produces a pair of non-identical interpolated signals F.sub.1 and F.sub.2 as described above - by adding three-fourths of one line's amplitude to one-fourth of the adjacent line's amplitude, and vice versa. For high frequency signals (luma highs and chroma signal), however, the apparatus produces an identical line-combed output signal for both fields - i.e., F.sub.1 =F.sub.2 =1/2L.sub.1 +1/2L.sub.2.